Hierarchy of micro-arousals and the microstructure of sleep

This review is aimed at providing an overview concerning the hierarchy of d ifferent kinds of micro-arousals (without awakening) during slow wave sleep (SWS), and to summarize available data on the dynamic interplay of phasic events constituting the microstructural web of sleep EEG. K-complexes are c onsidered elementary forms of arousal during SWS. They carry characteristic s of evoked potentials, which provide subattentive information processing a nd have at the same time level-setting sleep maintenance functions. Micro-a rousals are more complex arousal-dependent phasic events in the hierarchy. One class of recurring micro-arousals are preceded by K-complexes, while ot hers, such as phases of spontaneous transitory activation - type micro-arou sals - represent higher levels of arousal and are associated with EEG desyn cronization, increased muscle activity and signs of autonomous arousal. All types of micro-arousals function in a complex interrelationship with anoth er phasic event - sleep spindles - interpreted as microstates inhibiting se nsory inflow through the thalamic relay system. Lastly the CAP (cyclic alternating pattern) phenomenon offers a global fram ework for characterizing and measuring arousal instability. Appearance of C AP sequences reflects arousal instability in a higher duration range than i ndividual micro-arousals. They represent an arousal control mechanism refle cting that all arousing influences set into motion an oscillatory level set ting system around the referential state providing a flexible adaptation fo r the system to defend it against perturbations. The whole arousal (without awakening) hierarchy thus seems to play an essential role in sleep regulat ion, serving both cyclicity and maintenance of sleep and providing at the s ame time flexible contact between sleeper and environment - preserving the possibility to wake up in case of any biological danger, and tailoring slee p program according to actual environmental or inner demands. Although at present there are no accepted rules for microstructural evaluat ion of sleep, microstructural aspects provide a more dynamic picture both a bout the preprogrammed and reactive changes in sleep. This approach gives u s some clues to better understand sleep disorders as well. Several studies concerning microstructural analysis of certain sleep disorders are also rev iewed (C) 1998 Elsevier, Paris.